Once again downpours and strong thunderstorms developed across the Philadelphia region on Wednesday, taking down trees and wires, flooding roads, and prompting several water rescues.
The storms targeted a corridor from Chester County, through part of Philadelphia and into South Jersey in areas still trying to dry out from last week’s soakings by Isaias and powerful storms on Friday. Flood warnings remained in effect until 6:45 p.m.
A flash-flood watch for the entire region remains until 11 p.m.
Before the rains even started, some of the region’s streams already were about up to here with all this rain and primed for another slosh-over, the weather service said. Rainfall last week in some areas incredibly was close to 1,000% of normal, according to the Middle Atlantic River Forecast Center.
What’s more, the atmosphere, possibly suffering from exhaustion, hasn’t been in a big hurry to move things along, so showers that formed could stuck in place for awhile, moving like a car with a flat tire.
Meanwhile, the weather service on Tuesday added one more to last week’s tornado count, confirming a twister that touched down in Milford, Sussex County, and crossed into Kent County. It was the third Delaware tornado of the week in a state that averages a grand total of one annually.
In all, the Mount Holly office confirmed eight Isaias-related tornadoes in its service territory, including one in Maryland. On Monday it added one that cut a nearly four-mile path in New Castle County on Friday, with a peak wind speed of 105 mph.
Why all the mayhem? In summer, the upper-air currents that steer weather systems can become quite lazy, so whatever happens tends to keep happening.
“You get stuck in a pattern,” said Valerie Meola, meteorologist at the Mount Holly office.
If you’re stuck in this one, it’s good to keep a poncho handy.
Last week’s countywide average rain totals look almost hallucinatory. Over eight inches fell upon Delaware County in the seven-day period that ended on Sunday. That was 947% of normal, according to the River Forecast Center in State College, Pa.
Chester County, where over 10 inches was measured in West Chester from Isaias and Friday’s outbreak, came in at 850% of normal.
Based on the flash-flood guidance, the weather service says, it wouldn’t take much to coax some streams out of their banks on Wednesday.
The soils are soaked.
The triggering mechanism for the expected storms is a cold front that has approached and, naturally, has stalled.
“Training and back-building storms are certainly possible,” the weather service said, and strong downward winds, or “downbursts,” are possible.
The favored area for storms appears to be Philadelphia and along the urban corridor, where runoff is a perennial issue.